The invention relates to a high-strength multi-phase steel.
The invention also relates to a method for producing a hot and/or cold rolled strip made of such a steel.
The invention relates in particular to steels with a tensile strength in the range of at least 750 MPa up to maximally 920 MPa with low yield to tensile ratios of maximally 73% for producing components, which have an excellent formability and improved welding properties, such as the failure of the welding seam.
The hotly contested market forces automobile manufacturers to constantly seek solutions for lowering fleet consumption, while retaining a highest possible comfort and vehicle occupant protection. In this context an important factor is on one hand to save weight in all vehicle components, however, on the other hand also a favorable behavior of the individual components when subjected to high static and dynamic stress during operation or in the case of a crash. Pre-material suppliers seek to address this requirement by providing high-strength and ultra-high-strength steels with thin sheet thickness to reduce the weight of the vehicle components while at the same time improving forming and component properties during manufacture and during use.
High-strength and ultra-high-strength steels enable lighter vehicle components, which leads to reduced fuel consumption and reduced pollution due to the reduced CO2 proportion.
These steels therefore have to meet relatively high demands regarding their strength and ductility, energy absorption capacity and during their processing, such as for example during pickling, hot or cold forming, welding and/or metallic coating (organic coating or varnishing).
Newly developed steels thus must met the demands placed on weight reduction, the increasing material demands on ultimate yield strength, strain hardening behavior and elongation at fracture while still being well formable, as well as the demands placed on the component of high tenacity, border crack resistance, energy absorption and strength and the bake hardening effect, but also improved suitability for joining in the form of e.g. improved weldability, such as improved failure behavior of the welding seam (fracture pattern).
Improved edge crack resistance means increased hole expansion capacity during forming and is also known as low edge crack (LEC) or high hole expansion (HHE).
Improved weldability is achieved inter alia by a lowered carbon equivalent. Synonymous terms for this are under-peritectic (UP) or the already known low carbon equivalent (LCE).
An improved failure behavior of the welding seam (fracture pattern) is achieved inter alia by the addition of micro-alloying elements.
The steel according to the invention also has the goal to selectively reduce the thickness of particular components made of micro-alloyed ferritic steels already used in automobile manufacturing in order to save weight.
Such a reduction of the thickness of the steel sheet thus requires a high-strength steel with single or multiphase microstructure in order to ensure sufficient strength of the motor vehicle components.
In vehicle construction dual phase steels are increasingly used, which consist of a ferritic basic structure in which a martensitic second phase is integrated. It has been shown that in low-carbon, micro-alloyed steels proportions of further phases such as bainite and residual austenite have an advantageous effect on the hole expansion behavior. Bainite can hereby be present in different manifestations.
The specific material properties of the dual-phase steels, such as for example a low yield-to-tensile ratio at very high tensile strength, strong strain hardening and good cold formability, are well known.
Generally the group of multi-phase steels is increasingly used, which include for example complex-phase steels, ferritic-bainitic steels, TRIP steels as wells as the above-described dual-phase steels, which are characterized by different microstructure compositions.
Complex-phase steels are according to EN 10346 steels that contain small proportions of martensite, residual austenite and/or perlite in a ferritic/bainitic basic structure, wherein a delayed re-crystallization or precipitations of micro-alloy elements cause a strong grain refinement.
Compared to dual-phase steels these complex-phase steels have higher yield strengths, a higher yield-to-tensile ratio, lower strain hardening and higher hole expansion capacity.
Ferritic-bainitic steels are according to EN 10346 steels, which contain bainite or strain-hardened bainite in a matrix of ferrite and/or strain hardened ferrite. The strength of the matrix is caused by a high dislocation density, by grain refinement and by the precipitation of micro-alloy elements.
Dual-phase steels are according to EN 10346 steels with a ferritic basic microstructure in which a martensitic second phase is incorporated island like, possibly also with proportions of bainite as second phase. Dual-phase steels possess high tensile strength while exhibiting a low yield-to-tensile ratio and strong strain hardening.
TRIP-steels are according to EN 10346 steels with a predominantly ferritic basic microstructure in which residual austenite is incorporated, which can transform into martensite during forming (TRIP effect). Due to its strong strain hardening the steel has very good uniform elongation characteristics and high tensile strength.
In connection with the Bake-Hardening effect, high component strengths can be achieved. These steels are suited for stretch forming and also for deep drawing. In the forming of the material however higher sheet holding forces and press forces are required. A relatively strong spring-back has to be taken into account.
The high-strength steels with single-phase microstructure also include for example bainitic and martensitic steels.
Bainitic steels are according to EN 10346 steels that are characterized by a very high yield strength and tensile strength at a sufficiently high expansion for cold forming processes. The chemical composition results in a good weldability. The microstructure typically consists of bainite. In some cases small proportions of other phases such as martensite and ferrite can be contained.
Martensitic steels are according to EN 10346 steels that as a result of thermo mechanical rolling contain small proportions of ferrite and/or bainite in a basic structure of martensite. This steel type is characterized by a very high yield strength and tensile strength at sufficiently high elongation for cold forming processes. Within the group of multi-phase steels the martenisititc steels have the highest tensile strength values.
The suitability for deep drawing is limited. The martensitic steels are predominantly suited for forming processes that involve bending, such as roll forming.
High-strength steels are used in structural components, chassis components and crash-relevant components as steel plates, Tailored Blanks (welded steel plates) as well as flexibly cold tolled strips, so called TRB®s.
The Tailor Rolled Blank lightweight construction technology (TRB®) enables a significant weight reduction as a result of the load-adjusted selection of sheet thickness over the length of the component or steel type.
In the continuous annealing facility, a special heat treatment is performed for setting a defined microstructure in which the steel is provided with its low yield strength by relatively soft components such as ferrite or bainitic ferrite and obtains its strength by its hard components such as martensite or carbon-rich bainite.
For economic reasons cold-rolled high-strength to ultra-high strength steel strips are usually subjected to recrystallizing annealing in the continuous annealing process to generate well formable steel sheet. Depending on the alloy composition and the strip cross section, the process parameters such as throughput speed, annealing temperature and cooling rate (cooling profiles), are adjusted corresponding to the demanded mechanical-technological properties by way of the microstructure required therefore.
For establishing a dual-phase microstructure, the pickled hot strip in typical thicknesses between 1.50 mm to 4.00 mm, or cold strip in typical thicknesses of 0.50 mm to 3.00 mm, is heated in the continuous annealing furnace to such a temperature that the required microstructure forms during the cooling. The same applies for configuring a steel with complex-phase microstructure, martensitic, ferritic-bainitic and also purely bainitic microstructure.
Especially in the case of different thicknesses in the transition region of one strip to another, a constant temperature is difficult to achieve. When annealing alloy compositions with too narrow process windows this can lead to the fact that for example the thinner strip is either moved through the furnace too slowly, thereby lowering productivity, or that the thicker strip is moved through the furnace too fast and the required annealing temperatures and cooling gradients for achieving the desired microstructure are not reached. This results in increased waste.
Widened process windows are necessary in order to enable the demanded strip properties at same process parameters also in the case of greater differences in cross section of the strips to be annealed.
The problem of a too narrow process window is especially pronounced in the annealing treatment when stress-optimized components made of hot or cold strip are to be produced, which have sheet thicknesses that vary across the strip length and strip width (for example as a result of flexible rolling).
However, when strongly varying sheet thicknesses are involved, production of TRB®s with multi-phase microstructure employing the presently known alloys and available continuous annealing systems requires increased costs, for example an additional heat treatment prior to the cold rolling. In regions of different sheet thickness, i.e., in case of varying degrees of rolling reduction, a homogenous multi-phase microstructure cannot be established in cold-rolled and hot-rolled steel strips due to the temperature difference in the conventional alloy specific narrow process windows.
A method for producing a steel strip with different thickness across the strip length is for example described in DE 100 37 867 A1.
When high demands on corrosion protection require the surface of the hot or cold strip to be hot dip galvanized, the annealing is usually carried out in a continuous annealing furnace arranged upstream of the hot dip galvanizing bath.
Also in the case of hot strip, depending on the alloy concept, the demanded microstructure is not established until the annealing in the continuous furnace, in order to realize the demanded mechanical properties.
Deciding process parameters are thus the adjustment of the annealing temperatures and the speed as well as the cooling rate (cooling gradient) in the continuous annealing because the phase transformation is temperature and time dependent. Thus, the less sensitive the steel is regarding the uniformity of the mechanical properties when the temperature and time course changes during the continuous annealing, the greater is the process window.
When utilizing the known alloy concept for a multiphase steel in the continuous annealing of hot and cold rolled steel strips of different thickness, the problem arises that although the alloy composition tested there satisfies the demanded mechanical properties, only a narrow process window is available for the annealing parameters to enable establishment of uniform mechanical properties across the strip length without having to adjust the process parameters.
When using the known alloy concepts for the group of multi-phase steels, the narrow process window makes it already difficult during the continuous annealing of strips with different thicknesses to establish uniform mechanical properties over the entire length and width of the strip.
In the case of flexibly rolled cold strip made of multi-phase steels of known compositions, the too narrow process window either causes the regions with lower sheet thickness to have excessive strengths resulting from excessive martensite proportions due to the transformation processes during the cooling, or the regions with greater sheet thickness achieve insufficient strengths as a result of insufficient martensite proportions. Homogenous mechanical-technological properties across the strip length or width can practically not be achieved with the known alloy concepts in the continuous annealing.
The goal to achieve the resulting mechanical-technological properties in a narrow region across the strip width and strip length through controlled adjustment of the volume proportions of the microstructure phases has highest priority and is therefore only possible through a widened process window. The known alloy concepts for multiphase steels are characterized by a too narrow process window and are therefore not suited for solving the present problem, in particular in the case of flexibly rolled strips. With the alloy concepts known to date only steels of one strength class with defined cross sectional regions (sheet thickness and strip width) can be produced, hence requiring different alloy concepts for different strength classes or cross sectional ranges.
The state of the art is to increase the strength by increasing the amount of carbon and/or silicone and/or manganese (solid solution hardening) and to increase strength via the microstructure adjustments at adjusted temperature profile.
However, increasing the amounts of the aforementioned elements increasingly worsens the material processing properties for example during welding, forming and hot dip coating, but also the industrial production in all process steps such as steel production, hot rolling picking, cold rolling and heat treatment with/without hot dip coating places increased demands on the individual facilities.
In the steel production there is a trend toward reducing the carbon equivalent in order to achieve an improved cold processing and better properties during use.
For describing and quantifying the strip processing, in particular the edge crack behavior, the hole expansion test according to ISO 16630 is used as one of multiple possible testing methods.
But also suitability for welding, characterized by the carbon equivalent has become an increased focus.
For example in the following carbon equivalentsCEV=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5CET=C+(Mn+Mo)/10+(Cr+Cu)/20+Ni/40PCM=C+(Mn+Cu+Cr)/20+Ni/60+Mo/15/+/V10+5Bthe characteristic standard elements such as carbon and manganese as well as Chromium or Molybdenum and Vanadium are taken into account.
Silicone plays a minor role for calculating the carbon equivalent. This is of deciding importance with regard to the invention. The lowering of the carbon equivalent through lower contents of carbon and manganese is to be compensated by increasing the silicone content. Thus the edge crack resistance and welding suitability are improved at same strengths.
A low yield-tensile ratio (Re/Rm) of below 65 is typical for a dual-phase steel and serves in particular for the formability in stretching and deep drawing processes. This provides the constructor with information regarding the distance between ensuing plastic deformation and failing of the material at quasi-static load. Correspondingly lower yield strength ratios represent a greater safety margin for component failure.
A higher yield to tensile ratio (Re/Rm) of over 65, as it is typical for complex-phase steels, is also characterized by a resistance against edge cracks. This can be attributed to the smaller differences in the strengths of the individual microstructure components, which has a positive effect on a homogenous deformation in the region of the cutting edge.
With regard to the yield strength the norms provide for an overlap region within which an attribution to complex-steels as well as dual-phase steels is possible.
The analytical landscape for achieving multi-phase steels with minimal strengths of 750 MPa is very diverse and reveals very broad alloy ranges regarding the strength-promoting elements carbon, silicone, manganese, phosphorous, aluminum and also chromium and/or molybdenum as well as regarding the addition of micro-alloys such as titanium niobium, vanadium and/or boron and regarding the material-characterizing properties.
The spectrum of dimensions is broad and lies in the thickness range of 0.50 to 4.00 mm. Predominantly strips of up to about 1850 mm are used but also slit strip dimensions, which are generated by longitudinally separating the strips. Sheets or plates are generated by transverse separation of the strips.